Cube is an open source multiplayer and singleplayer first person shooter game built
on an entirely new and very unconventional engine.
Cube is a landscape-style engine that pretends to be an indoor FPS engine, which
combines very high precision dynamic occlusion culling with a form of geometric mipmapping
on the whole world for dynamic LOD for configurable fps & graphic
detail on most machines. Uses OpenGL & SDL.

Allows in-engine editing of geometry in full 3D (you fly around the map,
point / drag stuff to select it / modify it), which can even be done
simultaneously with others in multiplayer (a first!). Has simplistic but effective
fine grain vertex lighting that looks like lightmapping and can do dynamic
lights & shadows. Doesn't need
any kind of map precompilation, even lighting is done on the fly.
Has very simplistic quad-tree world structure that can do
slopes (heightfields with caps) and slants, water, does decent collision detection & physics,
has client/server networking that goes a long way in giving a lag-free game experience,
and features a Doom/Quake-style singleplayer (2 game modes, savegames) and multiplayer (12 game modes, master server / server browser, demo recording) game with some uncompromising brutal oldskool gameplay.

Most of the engine design is targeted at reaching feature richness through
simplicity of structure and brute force, rather than finely tuned complexity.

You can always download the latest release in the file section. Officially supported platforms (client & server)
are win32, linux & bsd (x86), macosx (ppc) and pocketpc (xscale), and includes source code (open source: ZLIB license). A recent
readme is here that gives some more information.
Some screenshots.

The cube/Sauerbraten IRC channel is #sauerbraten on irc.quakenet.org.

Cube on SourceForge.
The Cube & Sauerbraten wiki.
Very old graphics engine stuff, some of which was the basis for cube.